GeSn Devices for Short-Wave Infrared Optoelectronics

The electronics industry has a large silicon infrastructure for the manufacture of complementary-metal oxide semiconductor (CMOS) based electronics. The increasing density of Si based circuits has set a pace that is now pushing the physical limits of connectivity between devices over conventional wire based links. This has driven the increasing interest in Si based optoelectronics and to use the groundwork already established by the electronics industry for lower cost optical communications. The greatest limitation to this effort has been the incorporation of a Si based laser, which requires integration of a direct bandgap material within this CMOS process. The Ge1-xSnx alloy is one material of interest for this field of Si photonics due to its compatibility on Si CMOS circuits and its direct bandgap for increasing Sn content. The past decade of material development in this field has led to Ge1-xSnx films grown on Si with direct bandgaps. The work in this dissertation set out to develop Ge1-xSnx based optoelectronics operating in the short-wave infrared (SWIR) region. The fabrication methodology of Ge1-xSnx active photonic components such as microdisk resonators, photoconductors, and avalanche photodiodes were developed. A simple, one-mask fabrication method was developed to create Ge1-xSnx microdisk resonators on Si, which could serve as a platform for the first on-Si CMOS laser. A study of the noise levels, effective carrier lifetime, and specific detectivity was conducted for the first time on any Ge1-xSnx detector. A systematic study of detectors with Sn content ranging from 0.9 to 10.0% were fabricated and measured for their responsivity and spectral response in the SWIR. A record high responsivity of 1.63 A/W was measured at the 1.55 μm wavelength for a 10% Sn photoconductor at reduced temperature. A long-wavelength cut-off for this device was measured out to 2.4 μm. Avalanche photodiodes were also developed and tested for devices with Ge1-xSnx absorption regions. The low noise operation and high responsivity of these detectors yield a detectivity that is comparable with commercially available detectors. This work established the baseline performance for this technology and demonstrates this material can be used for Si based optoelectronics

GeSn Devices for Short-Wave Infrared Optoelectronics

The electronics industry has a large silicon infrastructure for the manufacture of complementary-metal oxide semiconductor (CMOS) based electronics. The increasing density of Si based circuits has set a pace that is now pushing the physical limits of connectivity between devices over conventional wire based links. This has driven the increasing interest in Si based optoelectronics and to use the groundwork already established by the electronics industry for lower cost optical communications. The greatest limitation to this effort has been the incorporation of a Si based laser, which requires integration of a direct bandgap material within this CMOS process. The Ge1-xSnx alloy is one material of interest for this field of Si photonics due to its compatibility on Si CMOS circuits and its direct bandgap for increasing Sn content. The past decade of material development in this field has led to Ge1-xSnx films grown on Si with direct bandgaps. The work in this dissertation set out to develop Ge1-xSnx based optoelectronics operating in the short-wave infrared (SWIR) region. The fabrication methodology of Ge1-xSnx active photonic components such as microdisk resonators, photoconductors, and avalanche photodiodes were developed. A simple, one-mask fabrication method was developed to create Ge1-xSnx microdisk resonators on Si, which could serve as a platform for the first on-Si CMOS laser. A study of the noise levels, effective carrier lifetime, and specific detectivity was conducted for the first time on any Ge1-xSnx detector. A systematic study of detectors with Sn content ranging from 0.9 to 10.0% were fabricated and measured for their responsivity and spectral response in the SWIR. A record high responsivity of 1.63 A/W was measured at the 1.55 μm wavelength for a 10% Sn photoconductor at reduced temperature. A long-wavelength cut-off for this device was measured out to 2.4 μm. Avalanche photodiodes were also developed and tested for devices with Ge1-xSnx absorption regions. The low noise operation and high responsivity of these detectors yield a detectivity that is comparable with commercially available detectors. This work established the baseline performance for this technology and demonstrates this material can be used for Si based optoelectronics

Non-Equilibrium SiGeSn Group IV Heterostructures and Nanowires for Integrated Mid-Infrared Photonics

Le développement des nouvelles générations de dispositifs électroniques devient de plus en plus limité par la chaleur générée par effet Joule dans les puces électroniques à haute densité. Des circuits photoniques intégrés sur silicium (Si) compatibles avec les procédés CMOS ont été proposés comme solution rentable pour réduire le réchauffement des dispositifs tout en améliorant leur performance globale. Cependant, les émetteurs à base de Si sont jusqu’à présent les composantes les plus difficiles à concevoir pour ces circuits photoniques intégrés. La principale raison est la bande interdite indirecte qui limite sévèrement l’efficacité de la luminescence du Si. Récemment l’incorporation de l’étain (Sn) dans des alliages silicium-germanium représente une nouvelle direction de recherche qui mènera à des semiconducteurs de groupe IV ayant une bande interdite directe. Les semiconducteurs obtenus Ge1-x-ySixSny sont des alliages ternaires du groupe IV compatibles avec la technologie CMOS, et peuvent avoir une bande interdite directe ajustable en fonction de la composition et de la contrainte. Ces propriétés ont généré un grand intérêt pour développer ces semiconducteurs et mieux comprendre leurs propriétés. Dans cette perspective, ce travail présente une étude détaillée de la structure de bande de l’alliage ternaire Ge1-x-ySixSny contraint et relaxé en utilisant une approche théorique fondée sur le modèle des liaisons fortes. Cette méthode est basée sur une évaluation précise des constantes de déformation de Ge, Si et α-Sn en utilisant une approche stochastique de Monte-Carlo ainsi qu'une méthode d'optimisation basée sur le gradient. De plus, une nouvelle approche d'évolution différentielle efficace est également développée pour reproduire avec précision les masses effectives expérimentales et les énergies de bandes interdites. Sur la base de ces approches, nous avons élucidé l'influence du désordre dans la structure crystalline, de la contrainte et de la composition sur l'énergie de bande interdite de Ge1-x-ySixSny. Quand 0 ≤x ≤0.4 et 0 ≤y ≤0.2, nous avons trouvé que la contrainte élastique réduit la concentration critique de Sn nécessaire pour obtenir un semiconducteur à bande interdite directe avec des énergies de bande interdite correspondantes inférieures à 0.76 eV. Cette limite supérieure diminue à 0.43 eV pour les alliages ternaires à bande interdite directe complètement relaxés. La transition obtenue vers la bande interdite directe en fonction de la composition est décrite par y> 0.605x + 0.077 et y> 1.364x + 0.107 respectivement pour les alliages contraints et complètement relaxés. Les effets de la contrainte, à une composition fixe, sur la transition de bande interdite indirecte à directe ont également été étudiés et discutés.----------Abstract Progress in electronic devices has been increasingly limited by the heat generated due to Joule effect in high density electronic chips. Silicon (Si) integrated photonic circuits compatible with CMOS processing has been proposed as a viable solution to reduce the heating of devices while improving their overall performance. However, Si-based emitters are, until now, the most difficult components to design for these integrated photonic circuits. The main reason is the indirect band gap which severely limits the efficiency of Si emission and absorption of light. Recently, the incorporation of tin (Sn) into silicon-germanium alloys has been proposed to overcome this fundamental limit. The obtained semiconductors are Ge1-x-ySixSny ternary alloys of Group IV elements compatible with CMOS technology, and may have a band gap that is adjustable depending on the composition and the strain. These properties have generated a great interest to grow these semiconductors and to better understand their optoelectronic and physical properties. With this perspective, this work outlines detailed investigations of the band structure of strained and relaxed Ge1-x-ySixSny ternary alloys using a semi-empirical second nearest neighbors tight binding method. This method is based on an accurate evaluation of the deformation potential constants of Ge, Si, and a-Sn using a stochastic Monte-Carlo approach as well as a gradient based optimization method. Moreover, a new and efficient differential evolution approach is also developed to accurately reproduce the experimental effective masses and band gaps. Based on this, the influence of lattice disorder, strain, and composition on Ge1-x-ySixSny band gap energy and its directness were elucidated. For 0 ≤x ≤0.4 and 0≤y≤0.2, tensile strain lowered the critical content of Sn needed to achieve a direct band gap semiconductor with the corresponding band gap energies below 0.76 eV. This upper limit decreases to 0.43eV for direct gap, fully relaxed ternary alloys. The obtained transition to direct band gap is given by y>0.605x+0.077 and y>1.364x+0.107 for epitaxially strained and fully relaxed alloys, respectively. The effects of strain, at a fixed composition, on band gap directness were also investigated and discussed. Next, building upon the acquired knowledge from the band structure calculation, the analysis was extended toward quantifying the electron and hole confinement in a Ge1-ySny/Ge core/shell nanowire system. For that purpose, the conduction and valance band offsets were evaluated

Si-Based Germanium Tin Photodetectors for Infrared Imaging and High-Speed Detection

Infrared (IR) radiation spans the wavelengths of the windows: (1) near-IR region ranging from 0.8 to 1.0 μm, (2) shortwave IR (SWIR) ranging from 1.0 to 3.0 μm, (3) mid-wave IR (MWIR) region covering from 3.0 to 5.0 μm, (4) longwave IR (LWIR) spanning from 8.0 to 12.0 μm, and (5) very longwave IR extending beyond 12.0 μm. The MWIR and LWIR regions are important for night vision in the military, and since the atmosphere does not absorb at these wavelengths, they are also used for free-space communications and astronomy. Automotive and defect detection in the food industry and electronic circuits also use IR detection as non-contact inspection methods. IR detection is also applied in the medical field. The market of SWIR and MWIR detectors is primarily dominated by mature technology from III-V systems such as indium gallium arsenide (InGaAs and extended InGaAs), indium antimonide (InSb), from II-VI such as mercury cadmium telluride (MCT), lead sulfide (PbS), and from group IV such as silicon (Si) and germanium (Ge) for shorter wavelength. However, the mature IR photodetector technology is expensive, demands to operate at low temperatures, and has complicated fabrication processes. In order to lower cost by mass production, many approaches have been developed towards the hybrid integration of III-Vs or II-VIs on a Si substrate. At the same time, it is desirable to develop an alternative material to reduce the cost and improve the performance for high-temperature operations. The discovery of group IV (Si)GeSn alloys has opened a route for a new generation of IR detectors. The work in this dissertation set out to develop Si-based Ge1-xSnx photodetectors for low-cost infrared imaging and high-speed detection. A study of effective carrier lifetime and optical properties of Ge1-xSnx materials is presented. The carrier lifetime is then applied to model the Ge1-xSnx photodetectors. For optical properties of Ge1-xSnx materials, two empirical formulae with extracted constants and coefficients were developed: (1) Absorption coefficient. The absorption regarding Urbach tail, indirect and direct bandgap transitions were comprehensively considered; (2) refractive index. The developed formulae could simplify the optoelectronic device design process due to their parameter-based expressions. A comprehensive study of Si-based GeSn mid-infrared photodetectors is carried out. A set of photoconductors with Sn compositions ranging from 10.5% to 22.3% show the cutoff wavelength to be extended to 3.65 μm. The devices’ peak D* is comparable to that of commercial extended-InGaAs detectors. The GeSn photodiodes are also explored with an in-depth analysis of a dark current. The dark current is suppressed as the photodiode was passivated. Moreover, mid-infrared images were captured using GeSn photodetectors, showing the comparable image quality with that acquired by using commercial PbSe detectors. The performance of GeSn photodiodes with 6.44 % and 9.24 % Sn is evaluated under high-speed measurements and simulations. The cutoff wavelength is extended up to 2.2 μm and 2.5 μm for 6.44 % and 9.24 % Sn devices, respectively. The photodiodes’ bandwidth is 1.78 GHz, and the simulation shows excellent agreement with measurement results

Raman Spectroscopy Studies of Sn Ge and x 1-x ZnO:Mn Semiconductor Solid Solutions

Doctoral Program in Physics (MAP-Fis)O foco do trabalho apresentado nesta tese é o estudo por espectroscopia Raman de duas promissoras soluções sólidas de semicondutores (SSs), GeSn e ZnO: Mn. Estes dois sistemas são de grande interesse científico devido às suas potenciais aplicações em várias áreas, como a fotónica, optoeletrónica, eletrónica e spintrónica. O sistema GeSn é considerado o melhor candidato para a obtenção de semicondutores de banda proibida directa, compatível com a tecnologia Si CMOS, uma vez que a adição de estanho à rede de Ge deve diminuir a energia da banda de condução no ponto Γ da primeira zona de Brillouin. A solução sólida ZnO:Mn é outro material interessante devido à sua transparência na região visível do espectro eletromagnético, juntamente com a possibilidade de controlo da condutividade elétrica e eventualmente das suas propriedades magnéticas. Neste trabalho, as amostras em estudo foram produzidas usando duas técnicas de deposição diferentes, nomeadamente, epitaxia de feixe molecular (MBE) para o sistema GeSn (incluindo amostras de referência de germânio puro) e pulverização reativa por magnetrão para o ZnO:Mn (incluindo ZnO não dopado e ZnO dopado de Al). Estas técnicas de crescimento são brevemente explicadas na tese, juntamente com os métodos de caracterização primários utilizados, como Rutherford Backscattering Spectrometry (RBS), Microscopia de Força Atómica (AFM), Microscopia Electrónica de Transmissão (TEM) e Difração de Raios-X (XRD). Adicionalmente foi realizada espectroscopia de rotação de Faraday e medidas de Ressonância Magnética para algumas amostras do sistema ZnO:Mn que apresentaram um comportamento paramagnético à temperatura ambiente. A espectroscopia Raman é uma técnica poderosa para estudar materiais semicondutores. Esta técnica foi, nesta tese, utilizada para estudar as duas famílias de soluções sólidas, tanto do ponto de vista macro- como microscópico. Esta técnica permite a determinação de frequências de vibração atómicas e permite também analisar a simetria dos cristais, efeitos de desordem nas ligas, estados de superfícies e de interfaces, presença de deformações, impurezas, defeitos, etc. Esta capacidade torna a espectroscopia Raman a técnica de análise a escolher para estudo de semicondutores. Os fundamentos desta técnica aplicada a semicondutores e as suas características para soluções sólidas são discutidos em detalhes na tese. Uma característica comum a ambos os sistemas em estudo é a dopagem com um elemento do mesmo grupo químico (Sn / Ge e Mn / Zn). Muito embora o dopante seja do mesmo grupo o limite de solubilidade não é alto, ≈ 10% para Mn em ZnO e apenas ≈ 0,5% para estanho em germânio (embora seja possível cultivar amostras com maior conteúdo por deposição de baixa temperatura). A diferença de raio atómico entre os átomos dopantes e nativos perturba o comprimento da ligação que envolve um átomo dopante e, consequentemente, a sua força para ambas as soluções sólidas. No entanto, há uma diferença importante entre os dois sistemas, relacionada com a diferença de massa atómica entre o dopante e o hospedeiro. Consequentemente, um átomo de Mn inserido na sub-rede catiónica do cristal de ZnO origina um modo de vibração local, enquanto o dopante de Sn que substitui o átomo de Ge produz um modo de ressonância que se sobrepõe ao contínuo de fonões do cristal de Ge. Consequentemente, os modos relacionados com o estanho são intrinsecamente fracos na dispersão Raman em soluções sólidas de GeSn. O efeito da dopagem com Sn nas vibrações da estrutura Ge é devido (i) à diferença de massa atómica e (ii) à deformação macroscópica induzida pela força elástica (tensão) devido à diferença da constante de rede entre a camada da liga e o substrato. Os resultados de espectroscopia Raman obtidos estão de acordo com os modelos microscópicos calculados com os potenciais semi-empíricos de Tersoff e mostram que os efeitos (i) e (ii) compensam-se parcialmente numa camada sob tensão, de modo que o pico de Raman correspondente à vibração fundamental Ge-Ge se aproxime da sua posição espetral num cristal de germânio puro. Se a rede cristalina da solução sólida se relaxar, a tensão desaparece, em particular, devido à formação de deslocações na interface e o pico Raman desloca-se para menores números de onda. Os estudos de espectroscopia Raman do sistema ZnO:Mn, realizados com excitação não ressonante e ressonante, revelaram vários efeitos: (1) elevada intensidade da banda de fonões LO, sob excitação ressonante, atribuída à dispersão do tipo Frőhlich que é proibida nas condições fora de ressonância, acompanhada pelo (2) aparecimento de múltiplos fonões LO nos espectros Raman ressonante e (3) pela presença de um modo relacionado com Mn, visto unicamente em condições de Raman não ressonante, cuja intensidade aumenta com a concentração de Mn. Esses efeitos foram explicados teoricamente. As linhas intensas de multi-fonões LO observadas nos espectros de Raman ressonante, em todas as amostras de ZnO estudadas, são causadas por emissão assistida por fonões e reabsorção de fotões por excitões quentes com energia apropriada. Este efeito pode ser chamado de efeito pseudo-Raman. A natureza do modo relacionado com Mn (3) foi esclarecida com base nos cálculos do modelo da frequência de vibração local dos átomos de Mn na aproximação do defeito isótopo.This work is focused on Raman spectroscopy studies of two promising semiconductor solid solutions (SSs), GeSn and ZnO:Mn. Both systems are of high scientific interest owing to their potential applications across multiple areas, such as photonics, optoelectronics, electronics and spintronics. The GeSn system is considered the best candidate for direct band gap semiconductor compatible with Si CMOS technology, since the adding of tin is supposed to lower the conduction band energy in the Γ point of the Brillouin zone. The ZnO:Mn solid solution is another interesting material owing to its transparency in the visible range along with controllable electrical conductivity and eventually magnetic properties at room temperature. In this work, samples of the two materials were grown using two different deposition techniques, namely, molecular beam epitaxy (MBE) for the GeSn system (including reference samples of pure germanium) and magnetron sputtering for the ZnO:Mn one (including undoped and Al-doped ZnO). These growth techniques are briefly explained in the thesis along with the primary characterisation methods used, such as Rutherford Backscattering (RBS), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), and X-Ray Diffraction (XRD). Additionally, Faraday rotation spectroscopy and Magnetic Resonance measurements were performed for some samples of the ZnO:Mn system that showed a paramagnetic behaviour at room temperature. Raman spectroscopy, a powerful technique for studying semiconductor materials, was used to study the solid solution materials from both macro- and microscopic points of view. It allows for the determination of atomic vibration frequencies and permits to analyse the crystal symmetry, alloy disorder, the state of surfaces and interfaces, the presence of strain, impurities, defects, etc. This capacity makes the Raman spectroscopy a technique of choice in the study of semiconductors. The fundamentals of this technique applied to semiconductors and its features for solid solutions are discussed in detail in the thesis. A common feature of both alloy systems is the substitutional doping with an element of the same chemical group (Sn/Ge and Mn/Zn). Even though, the solubility limit is not high, ≈ 10% for Mn in ZnO and only ≈ 0.5% for tin in germanium (although it is possible to grow samples with higher content by low-temperature deposition). The difference in atomic radius between the substitutional dopant and host atoms perturbs the length of the bond involving a doping atom and, consequently, its strength for both solid solutions. Yet, there is one important difference between the two systems, related to the atomic mass difference between the dopant and the host. Consequently, a Mn atom inserted into the cationic sublattice of the ZnO crystal originates a local vibrational mode, while the Sn dopant substituting Ge produces a resonant mode overlapping with the continuum of Ge crystal phonons. Consequently, tin-related modes are intrinsically weak in Raman scattering by GeSn SSs. The Sn doping effect on the Ge lattice vibrations is caused by (i) the atomic mass difference and (ii) a macroscopic strain induced by the elastic force caused by the lattice constant difference between the alloy layer and the substrate. The Raman spectroscopy results are in agreement with those of microscopic modelling performed using the semi-empirical Tersoff potentials and show that the effects (i) and (ii) partially compensate each other for a strained layer, so that the Raman peak is redshifted for relaxed alloys where the stress vanishes, e.g. because of the formation of misfit dislocations. Raman spectroscopy studies of the ZnO:Mn system, performed with either nonresonant or resonant excitation, revealed several effects: (1) strong enhancement of the LO-phonon band attributed to the “forbidden” Fröhlich-type scattering that becomes dominating under resonance conditions, accompanied by (2) the appearance of multiple LO-phonon peaks, and (3) the presence of a Mn-related mode, seen only off resonance, with the intensity increasing with the Mn content. These effects have been explained theoretically. The intense multi-LO-phonon lines observed in the resonant Raman spectra of all studied ZnO samples are caused by phonon-assisted emission and reabsorption of photons by hot excitons with appropriate energy, which may be called a pseudo-Raman effect. The nature of the Mn-related mode (3) was clarified based on model calculations of the local vibration frequency of Mn atoms beyond the isotope defect approximation

Quantum well based group-IV SiGeSn semiconductor laser and optoelectronic devices

Group IV photonics is attracting more and more attention these days in order to realise large scale optoelectronic integration. Although many prototype devices have been demonstrated, few of them can be put into practical applications. One of the major challenges is the lack of the optimized design and theoretical models. In this project, a computational model based on 8-band \textbf{k.p} theory is built to simulate the Group IV quantum well based semiconductor, and in this thesis it is applied to two different types of devices, lasers and modulators. For lasers, we focus on the tensile strained structure as it can reduce the Sn content needed to reach direct bandgap structure, moreover, because of the splitting of light and heavy holes, the low density of states at the valence band top will be beneficial to lasing. Since Ge has indirect band gap, and the L valley may still be close to $\Gamma$ valley even for direct gap GeSn, effective mass method is used for electrons in the L-valleys. The optimal range of well widths for transverse magnetic (TM) gain is found to be around 13-16nm. With constant well width (14 nm), the optimal choice of both inter-band gain and net gain with varying carrier density for different photon transition energy is found, by doing calculations throughout the parameter space of Sn and strain in the well. The inter-valence band absorption with split-off band was found to be an important loss mechanism that seems to be rarely discussed in the literature. A large inter- valence band absorption was found around 0.4-0.5 eV, when the bandgap is equal to the difference between the top valence band and split off band. And because of the influence of such loss mechanism the optimal choices of net gain have changed from that of the inter-band gain. Using the optimal Sn content and strain combined with waveguide design, the threshold current was estimated to be 1.19 kA/cm$^{-2}$, comparable to conventional III-V quantum well lasers. For modulators, a novel way using an intra-step quantum well was applied here to improve the performance of the GeSn quantum well electroabsorption modulator. Using SiGeSn as the barrier and GeSn as the material for the well layers, an intra-step well can be made by using different Sn contents in two intra-layers. The band structure is also calculated by the \textbf{k.p} method, and the exciton effect was considered by variational method. Without increasing the total well width, and compared to the square quantum well, a much larger quantum confined Stark effect can be realised with intra-step quantum well. By considering the figures of merit related to practical performance, $\Delta\alpha/F$ and $\Delta\alpha/F^2$, the intra-step quantum well, compared to square quantum well, brings about 44\% improvement on the bandwidth per unit applied voltage and 46\% reduction on the power consumption per bit data transmitted. The model presented in this thesis can still be improved. It can include other realistic effects such as carrier transport, and on the other hand, the accuracy can also be improved by considering `better' choice of parameters and perhaps higher order \textbf{k.p} theory. Other applications are also possible based on the existing code, e.g. light emitting diodes and photodetectors

Electrical Characterization of Fabricated pin Diodes made from SixGe1-x-ySny with an Embedded Ge1-xSnx Quantum Well

The interest in group-IV based optoelectronic devices has increased due to a foreseeable future demand. The main advantage is the relatively simple integration into the modern Silicon-based Complementary Metal-Oxide-Semiconductor technology platform. The ternary alloy Silicon-Germanium-Tin enables achieving a direct bandgap material made from the indirect semiconductors Silicon, Germanium and Tin. By using a virtual Germanium substrate technology these semiconductors can be integrated on a Silicon wafer. In this paper, we discuss the characterization of grown and fabricated pin-diodes made from the ternary alloy Silicon-Germanium-Tin by using Molecular Beam Epitaxy technology on a virtual Germanium substrate formed on Silicon(001) wafers. To achieve higher Tin concentrations to enable direct band transitions, a thin Germanium-Tin layer is inserted into the intrinsic region of the pin-diodes resulting in a quantum well. It is shown that these pin-diodes have electrically good characteristics and in particular a low dark current density, which suggest a high crystal-quality